Mere reference to background art herein should not be construed as an admission that such art constitutes common general knowledge or admissible prior art in relation to this application.
Blood pressure is a fundamental cardiovascular parameter that represents the force that drives perfusion of the body and in part reflects the workload of the heart [1]. There are difficulties in obtaining a meaningful reading of blood pressures; first, different techniques produce significantly different readings and second, blood pressure can change significantly due to environmental or emotional changes. To better utilise blood pressure information to interpret patient events, clinicians require blood pressure trend information preferably combined with other relevant physiological measures of the patient such as capnography. At present, blood pressure trend information is either gathered by invasive measures that provide continuous information or through non-invasive measurements of blood pressure (NIBP) that provide intermittent information.
Interpreting the different measurements of blood pressure in relation to other physiological parameters can be difficult due to the low rate of sampling of the visually displayed information by the clinicians. This is especially problematic when using automatic NIBP, as the time elapsed since the last measurement might be unknown, and it is not immediately apparent whether a loss of information is due to failure within the machine or the patient [1].
Blood pressure is normally represented using numerical and graphical visual displays, and abnormal states may be indicated with auditory alarms. There are a range of proposed displays that integrate data, including blood pressure, graphically in a way that shows higher order properties of the anesthetized patient's state [2]-[4]. These displays are dependent upon the clinician's ability to attend to the visual display, which is not always possible [5]. Further, these displays do not convey blood pressure in the auditory modality nor do they distinguish between intermittent and continuous measurements of physiological parameters. There is strong evidence showing that auditory alarms are not particularly effective at directing clinicians' attention to visual displays [5]-[7]; therefore, even if the proposed visual displays are of benefit in providing integrated data, it is likely that people in care-providing environment are likely to miss some important information on the visual displays.
Other approaches have attempted to sonify blood pressure as part of a multi-parameter continuous auditory display [8]. These sonifications have at least four problems. First, the blood pressure information may not be continuously measured and therefore the inclusion of such a parameter may be misleading as the blood pressure may have changed since the last value was measured. This is potentially very dangerous since clinicians' diagnoses may be made on the basis of incorrect information [5]. Second, the blood pressure information has to be separated out from the sonification by focusing on one or more sound dimensions. Third, no historical information is included in the sonification. Fourth, the sonification has not been designed to support integration across the visual and auditory modalities.
U.S. Pat. No. 6,947,780 discloses method and apparatus for sonification of physiological data, and in particular blood oximeter readings. An audio signal is generated at each pulse, dependent upon the measured oximeter reading. If the measurement corresponds to one of a plurality of pre-determined transition points in a range, a tone of a respective frequency is generated. For readings which fall between a pair of the transition points, a dual tone signal is generated. The dual tone signal comprises two frequencies, each of which has its amplitude modified by a respective factor which depends upon the proximity of the reading to the pair of transition points. Although no evaluation data is presented for this system of sonification, it seems likely that clinicians would find it difficult to distinguish between tones of closely spaced frequencies and between different amplitudes, particularly at typical pulse rates.
International patent application WO 03/017838 discloses sonification of respiratory behaviour. That method uses tones of different pitch to represent different levels of measured respiratory parameters, e.g. respiratory flow and carbon dioxide concentrations. A clinician or other user must distinguish aurally between parameter values on the basis of pitch alone.
It is an aim of the present invention to provide an improved method and apparatus for monitoring blood pressure, which overcome or alleviate one or more of the limitations and disadvantages of prior art blood pressure monitoring systems, particularly during anaesthesia and intensive care, or which at least provide the public with a useful choice.